City gauges water needs with simulation software

Rocky Mount, N.C., has begun using risk management techniques to preserve its water supply and to avoid the cost of building an additional reservoir. By balancing the needs of residents with the high costs of a fail-proof system, the city has delayed a $70 million expansion of its water system.

In 1993, Rocky Mount suffered through a drought that was the worst on record at the time. Although it did not prompt emergency measures, the drought was a wake-up call to the city that took for granted its reliable water supply. The city began considering new sources of water, including building a second reservoir and raising an existing dam. The most feasible choice for providing a fail-proof supply involved pumping raw water and treated wastewater long-distance from and to the Roanoke River. Because that $70 million option was cost-prohibitive, the city hired Columbia, Md.-based HydroLogics, a water resources management firm, to investigate the efficacy of more intensive management of the system.

Using its OASIS simulation software, the company developed a drought management tool for the city. The tool generates equally likely inflow forecasts based on recent soil moisture conditions. Water system operation is simulated with each of the forecasts, and the probability of reservoir storage levels at various times in the future is assessed. If the possibility of system failure is too great, the simulations can be re-run using a different minimum release regime and/or a reduced demand level to determine when (or if) demand reductions should be imposed and what their magnitude should be.

Through such model simulations, Rocky Mount created a drought management policy that balanced the risks of system failure and the hardship from the imposition of demand restrictions. As part of the plan, North Carolina also approved a minimum release reduction from the reservoir concurrent with the city’s implementation of conservation measures.

In September 1999, the city was confronted with an entirely different problem. Hurricane Floyd dropped 25 inches of rain on the city, causing extensive flooding. As a result, the city was able to acquire two abandoned rock quarries for $625,555 with 70 percent assistance from the FEMA Hazard Mitigation Program, which provides grants to local governments for the purchase of flood-ravaged property.

The quality of water in the quarries met state standards, allowing the city to pump from the quarries to supplement its supply. Model simulations indicated that with the additional quarry supply, the city would be able to reliably meet demands through the year 2050.

In 2002, Rocky Mount faced another drought. Fortunately, planning for activating the quarries was under way. The city used simulations to determine when to begin pumping operations and when to implement conservation measures. Pumping began in early August 2002 and ended about six weeks later, at a total out-of-pocket cost of less than $40,000. As a result, the city was able to weather the last year of a region-wide, four-year drought with relative ease. Other communities in North Carolina were not so fortunate and were forced to make expensive emergency interconnections with other systems.

The city found that risk management could be a far less expensive option than capacity expansion. By using multiple inflow forecasts to make operating decisions, Rocky Mount has gained confidence in its ability to manage future droughts, and it has deferred indefinitely the $70 million capacity expansion project. In addition, its risk management program has facilitated communication with affected parties, including the state, regarding the conditions under which the city would be allowed to reduce the minimum release from its reservoir.
— Paul Blount, director of water resources for Rocky Mount, N.C., and Peter Varney, assistant city manager.